1. Field of the Invention
The present invention relates to positioning systems for moving a member between positions in a minimum time.
2. Description of the Prior Art
A typical positioning application to which the present invention relates is the positioning of a data recording head over a selected track of a magnetic disk file. Typical contemporary file systems provide positioning in two independent modes of operation, i.e., a seek mode and a track follow mode. One such system for controlling a disk file head access operation is described in an article entitled "Design of a Disk File Head-Positioning Servo" by R. K. Oswald, (IBM Journal of Research and Development, Nov. 1974, pp. 506-512). For the seek mode, the primary requirement has been to effect movement from an initial track position to a target track position in a minimum time. This has been accomplished conventionally by means of a derived continuous distance-to-go signal acting on a reference velocity curve generator which, via a high gain closed loop, forces the actual velocity of the system to follow the reference velocity trajectory of the curve generator.
The seek mode, therefore, is necessarily of wide bandwidth and as such is subject to the stability and error constraints of such systems. Conversely, the track-follow system is of narrow bandwidth since the primary requirement is to "lock" the system onto the target track and thereafter minimize positioning error caused by low frequency disturbances such as runout or windage.
This dichotomy of requirements has necessitated design of two independent systems operating from the same position and velocity error source. The wide band requirements of the seek system have forced this error source to be continuous with the consequence that a servo position reference data area remote from the information recording area has been provided in the form of either a separate servo disk, as described in the above referenced Oswald article, or separate servo band. Clearly it is technically desirable to provide the servo data contiguous with the recorded data and to remove the necessity for a remote servo data area. This can be accomplished by interspersing servo data with recorded data and thereby invoking a sampled-data approach to positioning of the recording apparatus. However, since direct head position and velocity information is only available at servo data sampling times, sampled systems are inherently of low bandwidth. The conventional approaches for minimum time track seek motion described above are thus not applicable for sampled data systems.
One approach to the problem of making the performance of a sampled system approach that of a continuous system is shown in U.S. Pat. application Ser. No. 811,350 by R. K. Oswald and commonly assigned with the present application. The system described employs concurrent feedforward and feedback control to move a member between positions in time optimal fashion. The feedforward control system provides coarse control and the feedback control system provides fine control to correct for deviations between the actual and a nominal system. The feedback loop may thus be of low bandwidth, such as in a sampled system. The feedforward control function represents the approximate input to an electromagnetic coil actuator of a nominal system required to move the member between positions in time optimal fashion.
The problems of low bandwidth in feedback control are also known in the process control art where they have been tackled by the employment of model systems. In an article on p. 98 of Vol. 1 of "Nonlinear and Adaptive Control Techniques" (Purdue University), a process control system is shown which employs a model of the process, responsive to command signals to provide a forward control signal to a "nonlinear trajectory algorithm" block and to provide a "predicted process state." The trajectory is added to a feedback control signal, developed by comparing a "process state." The sum is applied as a "control output" to the process. The parameters of the model are updated in response to the "process state" measurement. U.S. Pat. Nos. 3,601,588 (Bristol), 3,657,534 (Bakke) and 3,758,762 (Littman) all show various process control systems employing process models, some adaptive, broadly resembling the system shown in the Purdue article.
Patents exist in the aircraft control art which show so called model systems for producing a predicted system response to a command. Typical of these are U.S. Pat. Nos. 3,137,462 (Hendrick) and 3,221,229 (Kezer). In all of these patents, the predicted system response is compared with the actual measured system response and the difference used to control the parameters of the control system adaptively. Quantities controlled include servo amplifier gain, stabilization and directional control systems for the control surfaces.